1. Field
Some embodiments generally relate to the collection and concentration of solar radiation. More specifically, embodiments may relate to systems to improve the durability and/or efficiency of solar radiation collectors.
2. Brief Description
A concentrating solar radiation collector may receive solar radiation (i.e., sunlight) over a first surface area and direct the received radiation to a second, smaller, surface area. Accordingly, the intensity of the solar radiation within the second area is greater than the intensity within the first area. Existing power systems may leverage this increased intensity to generate electricity in any number of ways.
For example, a conventional parabolic trough concentrator consists of a long trough-shaped mirror and a liquid-filled pipe located between the mirror and the sun. In operation, the mirror reflects and concentrates received solar radiation onto the liquid-filled pipe. The concentrated solar radiation heats the liquid, which may then be used to drive heat-powered electrical generators (e.g., steam turbines).
Co-pending U.S. patent application Ser. No. 11/138,666, entitled “Concentrator Solar Photovoltaic Array with Compact Tailored Imaging Power Units”, describes several types of solar power units utilizing unique collector configurations. FIG. 1A is a cross-sectional view of one example of the power units described therein. Power unit 10 includes primary mirror 11, secondary mirror 12, protective surface 13, housing 14, optical element 15 and photovoltaic cell 16.
FIG. 1A depicts “on-axis” operation of power unit 10, in which incoming radiation 20 is substantially normal to surface 13. As illustrated, radiation 20 passes through surface 13 and is received by primary mirror 11. Primary mirror 11 reflects received radiation 20 toward secondary mirror 12, which in turn reflects radiation 20 toward a desired focal area f. Focal area f may comprise a point or larger area on and/or within optical element 15. In this regard, optical element 15 comprises a tapered optical rod which directs concentrated radiation 20 received at focal area f to cell 16. Cell 16 then converts concentrated radiation 20 to electricity using known techniques.
In one variation of power unit 10, focal area f and an entry surface of optical element 15 are positioned closer to secondary mirror 12 than as illustrated in FIG. 1A. Secondary mirror may be substantially flat in other variations.
FIG. 1B depicts “off-axis” operation of power unit 10, in which incoming radiation 30 is not substantially normal to surface 13. According to some examples, power unit 10 exhibits off-axis operation in a case that incoming radiation 30 is more than 2° from normal to surface 13. As described above, radiation 30 passes through surface 13, is received by primary mirror 11, and is reflected by primary mirror 11 toward secondary mirror 12. However, due to the off-axis operation and the underlying geometric relationships of the elements of power unit 10, secondary mirror 12 reflects received radiation 30 toward an area of primary mirror 11 rather than to desired focal area f.
Exposure to concentrated radiation as illustrated in FIG. 1B may crack, deform or otherwise damage primary mirror 11. Such damage may reduce an efficiency and/or operational lifetime of power unit 10. To address this phenomenon, elements of concentrator 10 may be redesigned to reduce the ratio at which received radiation is concentrated. Unfortunately, such a redesign would also reduce the concentration ratio during on-axis operation, thereby reducing a maximum electrical output of power unit 10.